1. Field of the Invention
The present invention relates to a method of preparing a bismuth superconductor, and more particularly, it relates to an improvement for increasing the critical current density of a bismuth superconductor and homogenizing properties thereof.
2. Description of the Background Art
In recent years, ceramic superconducting materials, i.e., oxide superconducting materials, have been watched with interest as superconducting materials which exhibit higher critical temperatures.
In particular, yttrium, bismuth and thallium superconducting materials are expected for practical use with high critical temperatures of about 90 K, 110 K and 120 K respectively.
One of conventional methods of preparing oxide superconductors comprises the steps of mixing raw materials for forming an oxide superconductor with each other, heat treating the mixture, pulverizing the mixture, and thereafter covering the mixture with a metal sheath. This method is advantageously applied to preparation of a long superconducting wire, for example.
A superconductor to be applied to a cable or a magnet, for example, must have a high critical current density, in addition to a high critical temperature. In particular, it is necessary to ensure a required critical current density under a magnetic field which is applied to the superconductor. Further, properties such as the critical current density must be homogeneously attained over the superconductor. In a long superconducting wire, for example, such properties must be undispersed along its longitudinal direction.
However, a conventional oxide superconductor exhibits a rather insufficient critical current density particularly under a magnetic field, and has only unsatisfactory homogeneity of properties.